The discovery of three irregular satellites of Neptune was announced
Jan 13, 2002, by an international team of astronomers, including
Information on the discoveries and the tracking effort are provided below.
Discovery announcement Harvard-Smithsonian Center for Astrophysics
Discovery announcement National Research Council of Canada
Press Release (Jan 15/2003) University of British Columbia.
UBC astronomer Brett Gladman is part of an international collaboration which
has just announced the discovery of 3 new moons of the planet Neptune.
The team of astronomers is lead by
of the Harvard-Smithsonian Center for Astrophysics and
of the National Research Council of Canada.
This boosts the number of known satellites of Neptune to eleven.
These moons are the first to be discovered orbiting Neptune since the
Voyager II flyby in 1989, and the first discovered from a ground-based
telescope since 1949.
It now appears that each giant planet's irregular satellite population is the result of an ancient collision between a former moon and a passing comet or asteroid. "These collisional encounters result in the ejection of parts of the original parent moon and the production of families of satellites. Those families are exactly what we're finding," said Kavelaars.
The team that discovered these new satellites of Neptune includes Holman and Kavelaars, graduate student Tommy Grav (University of Oslo & Harvard-Smithsonian Center for Astrophysics), and undergraduate students Wesley Fraser and Dan Milisavljevic (McMaster University, Hamilton, Canada).
The new satellites were a challenge to detect because they are only about 30-40 kilometers (18-24 miles) in size. Their small size and distance from the Sun prevent the satellites from shining any brighter than 25th magnitude, about 100 million times fainter than can be seen with the unaided eye.
NEEDLE IN A HAYSTACK
To locate these new moons, Holman and Kavelaars utilized an innovative
technique. Using the 4.0-meter Blanco telescope at the Cerro Tololo
Inter-American Observatory, Chile, and the 3.6-meter Canada-France-Hawaii
telescope, Hawaii, they took multiple exposures of the sky surrounding the
planet Neptune. After digitally tracking the motion of the planet as it
moved across the sky, they then added many frames together to boost the
signal of any faint objects. Since they tracked the planet's motion, stars
showed up in the final combined image as streaks of light, while the moons
accompanying the planet appeared as points of light.
Prior to this find, two irregular satellites and six regular satellites of Neptune were known. The two irregular satellites were also the largest: Triton, discovered in 1846 by William Lassell, and Nereid, discovered in 1949 by Gerard Kuiper. Triton is considered irregular because it orbits the planet in a direction opposite to the planet's rotation, indicating that Triton is likely a captured Kuiper Belt Object. (The Kuiper Belt is a disk-shaped collection of icy objects that circle the Sun beyond the orbit of Neptune.) Nereid is considered irregular because it has a highly elliptical orbit around Neptune. In fact, its orbit is the most elliptical of any satellite in the solar system. Many scientists believe that Nereid once was a regular satellite whose orbit was disrupted when Triton was gravitationally captured. The six regular satellites were discovered by the Voyager probe during its encounter with Neptune. The three new satellites were missed by Voyager II because of their faintness and great distance from Neptune. According to Holman, "The discovery of these moons has opened a window through which we can observe the conditions in the solar system at the time the planets were forming."
TRACKING FAINT BLIPS
The researchers are currently conducting follow-up observations to better
define the orbits of the newfound moons using orbital predictions supplied
by Brian Marsden (Director of the Minor Planet Center in Cambridge, Mass.)
and Robert Jacobson (Jet Propulsion Laboratory).
To follow up the initial find, team member Brett Gladman (University of British Columbia, Canada) led a group of astronomers including Jean-Marc Petit, Philippe Rousselot, and Olivier Mousis (Observatoire de Besancon, France); and Philip Nicholson and Valerio Carruba (Cornell University) in conducting additional observations using the the European 8-meter ESO Very Large Telescopes in Chile and the Hale 5-meter telescope on Mount Palomar in California. Gladman says: "The tracking of these extremely faint objects is extremely difficult, but necessary for without many observations one cannot calculate their orbits around the planet in order to learn about their origin."
Like the orbits of other similar satellites, the preliminary orbits of S/2002 N1, S/2002 N2, and S/2002 N3 are large, significantly elliptical, and inclined with respect to the plane of the solar system. The orbits of S/2002 N2 and S/2002 N3 are prograde, while that of S/2002 N1 is retrograde. For orbit details, see the International Astronomical Union's Circular and the associated Minor Planet Electronic Circular. These new Neptunian moons appear to have inclinations closer to 90 degrees than those of other irregular satellites. This may reflect differences in the means by which these objects were captured are/or the smaller tidal forces from the sun for Neptune's distance and mass. Because the orbits of these moons may have been gravitationally perturbed by Triton's capture and subsequent orbital evolution, we can use these orbital parameters to make a preliminary investigation of the means and time scale of Triton's capture from heliocentric orbit.
The following images of the satellite S/2002 N 1 were acquired by P. Rousselot and O. Mousis at the ESO VLT UT-3 telescope on September 3rd 2002. Identification and astrometry (measurement of the position of the object on the sky) by B. Gladman.
False-colour image of S/2002 N 1 (false colour image) (.gif)
Black and white image of S/2002 N 1 (negative image) (.gif)
Black and white image of S/2002 N 1 (negative image) (.eps)